1. Field of the Invention
The present invention relates to a piezoelectric ceramic composition. More specifically, the invention relates to a piezoelectric ceramic composition that can be used as ceramic filters, ceramic resonators, ultrasonic oscillators, piezoelectric buzzers, piezoelectric ignition units, ultrasonic motors, piezoelectric fans, piezoelectric actuators, and as piezoelectric sensors such as acceleration sensors, knocking sensors and AE sensors. Particularly, the invention relates to a piezoelectric ceramic composition that can be preferably used as a second intermediate-frequency filter in a mobile communication unit such as analog cellular telephone, digital cellular telephone, etc.
2. Description of the Prior Art
Products utilizing a piezoelectric ceramic composition can be represented by ceramic filters, ceramic resonators, ultrasonic oscillators, piezoelectric buzzers, piezoelectric ignition units, ultrasonic motors, piezoelectric fans, piezoelectric sensors, piezoelectric actuators and the like.
Here, as the elements of ceramic filters and ceramic resonators, there has been used a ceramic composition comprising chiefly PbZrO.sub.3 --PbTiO.sub.3 which is blended with, or substituted with, a metal oxide such as Nb.sub.2 O.sub.5 or MnO.sub.2, or with a composite perovskite oxide such as Pb(Nb.sub.2/3 Mg.sub.1/3)O.sub.3 or Pb(Nb.sub.2/3 Co.sub.1/3)O.sub.3.
There has heretofore been known a composition Pb(Nb.sub.2/3 Co.sub.1/3)O.sub.3 --PbZrO.sub.3 --PbTiO.sub.3 as a piezoelectric ceramic composition having a large planar electromechanical coupling coefficient Kp and excellent piezoelectric property.
In recent years, furthermore, it has been urged to provide piezoelectric parts such as ceramic filters and ceramic resonators that can be used under various conditions such as being mounted on the surface of a substrate. Such piezoelectric parts are subject to be heated at from about 230.degree. C. to about 300.degree. C. when they are mounted on the substrate by reflow soldering. Therefore, these piezoelectric devices must have resistance against the heat.
In a filter designed to work on a band of, for example, several kilohertz by utilizing the planar vibration of a piezoelectric ceramics, furthermore, it becomes necessary to use a material having a small dielectric constant .epsilon.r and excellent heat resistance in order to increase the planar electromechanical coupling coefficient Kp, to decrease the mechanical quality factor, Qm and to increase the level at the defined frequencies. In a digital cellular telephone, in particular, it is required to stabilize the group delay time characteristics of a filter within a guarantee band in order to prevent the infiltration of noise from the neighboring channels.
However, the above-mentioned Pb(Nb.sub.2/3 Co.sub.1/3)O.sub.3 --PbZrO.sub.3 --PbTiO.sub.3 ceramic composition has a small resistance against the heat and causes the piezoelectric characteristics and resonant frequency to change to a large extent before and after the reflow soldering, which are problems from the practical point of view. Besides, this composition exhibits piezoelectric characteristics and resonant frequency that change to a great extent after the thermal shock test. When this composition is used for a filter in a communication unit mounted on a vehicle which will be subjected to severe environmental changes, it becomes difficult to maintain stable transmission and reception due to changes in the characteristics of the device.
Because of its dielectric constant .epsilon.r which is as large as about 1800, furthermore, the device must be provided with a partial electrode. Moreover, the large mechanical quality factor Qm deteriorates the group delay time characteristics.